We have used ultrasound treatment (1.9 MHz CW, 1.5 W/sq. cm SA in a water bath for 20 minutes) to treat transplantable tumors in mice. Ultrasound treatments were given immediately before or after X-ray. Three of the 4 tumors used (C3H mammary tumors, KHJJ and the EMT-6) showed no enhancement of radiation tumor damage with a reduced TCD50. The S-180 sarcoma, a highly radiosensitive tumor, showed a 37% reduction in the TCD50 for ultrasound before or after irradiation. Skin damage from the X-ray was more intense when radiation was given after ultrasound. Water bath heating (44 degrees C for 20 minutes) reduced the TCD50 for the S-180 tumor by an amount equal to that from comparable ultrasound. Experiments with pulsed ultrasound were as effective as with CW ultrasound or with the water bath. These data support a thermal hypothesis. Experiments are now underway to study ultrasound enhancement of X-ray damage to normal tissues.